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S. W. Designed in the 1970s to absorb excess nuclear production, the Coo pumped energy transfer station takes advantage of market volatility to operate increasingly powerful turbines.
Nestled among spruce trees, about fifty kilometers from the city of Liège, Belgium, the vast lakes of the Coo power plant - owned by the Electrabel group (Engie) - have recently encroached a few additional meters into the surrounding forest. "We kept the largest oak tree that oversaw our construction site, and we also created a one-hectare wetland where sheep graze," emphasizes Marc Locht, operation manager of the power plant.
For the uninitiated, the extension of one of the upper basins, completed in recent months, is not immediately obvious, as it already appears gigantic. "By raising the basin by 2 meters, we went from 4.1 million cubic meters to 5.1 million cubic meters of stored water," explains Marc Locht. Work is also underway a little lower, in the machine room. In this 40-meter-high cathedral, a brand-new rotor weighing nearly 300 tons awaits installation on one of the six hydroelectric plant turbines to increase its power.
In total, Engie plans to invest 67 million euros in this pumped energy transfer station (STEP) to increase its storage capacity and power by 7.5% by 2026.
The game is worth the candle: the facility, commissioned in 1967 with the initial goal of absorbing excess nighttime nuclear production and releasing it during the day, has found a new opportunity with the growth of renewable energies. "We are increasingly called upon to compensate for imbalances in the markets," explains Marc Locht.
Automatically controlled from Brussels, the Coo power plant basins are emptied and filled in sync with the rise and fall of wholesale electricity market prices, as intermittent wind and solar power plants nearby switch on and off.
A true trading tool, the plant has radically changed its pace: while its turbines used to start about 2,000 times a year in the 2010s, they now start 15,000 times a year. "We start each machine seven to eight times a day now, whereas before it was only in the morning and evening," confirms Marc Locht.
This start-and-stop operation has reduced the plant's overall electricity production but allows it to capture more value on the electricity wholesale markets, thanks to start-ups concentrated during periods of favorable volatility.
For European electricity network managers looking for new flexibility means, these "STEP" facilities have the advantage of providing very significant power in record time, an undeniable asset when weather fluctuations can radically change the situation from one hour to the next.
"During the storm this autumn, the winds were so strong that some wind farms in the North Sea suddenly stopped, instantly eliminating the need for 300 megawatts of power on the grid. The Coo plant then took over," explains Sébastien Arbola, Deputy CEO in charge of Engie's flexibility and retail activities.
Similar extension investments are planned by Engie for its Dinorwig and Festiniog STEP facilities in Wales. However, the group currently has no plans to invest in new assets of this type.
Complex to build, finance, and gain local acceptance, these power plants sometimes present a delicate economic equation. "These facilities are the holy grail... once they're built. We looked at projects in Italy and Australia, but we haven't decided to go there; it's a complex civil engineering issue," confirms Sébastien Arbola.
In addition, in France, there is an uncertain regulatory context, as EDF is working with the government to change the status of these assets, which have been operated under concessions until now. In the meantime, Engie is also focusing on batteries: it plans to install 10 GW worldwide by 2030, compared to barely 600 MW today.